The invention relates to a sensor element arrangement in a retaining frame for a pyrodetector which serves for the detection of a body entering into its detection area having a temperature deviating from its ambient temperature. The sensor element arrangement contains a concave mirror for focusing the heat radiation emanating from the body. A first sensor element is provided at the focus of the concave mirror, and two further sensor elements are provided for the compensation of environmental influence. A film of uniform electrically non-conductive material having a permanent isodirectional orientation polarization and with electrodes formed of thin metal layers situated on both sides of the film is provided for the first and for the two further sensor elements. The electrodes are connected to an electronic evaluation means via electrical lines. The sensor element arrangement is secured in a mounting in the concave mirror, and the film with the sensor elements is arranged in the concave mirror such that a radiation incidence of the radiation reflected in the concave mirror is present on both surface sides of the film. The concave mirror is a parabolic mirror dimensioned such that its focus lies in the inner half of the concave mirror. The film with the sensor elements lies on the optical axis of the concave mirror so that the first sensor element is essentially impinged only by radiation of the detectable body situated at a greater distance which has been reflected in the concave mirror.
The invention further relates to a method for the manufacture of a sensor element arrangement in a retaining frame. First the sensor elements are generated by appropriate metallization on a plastic film formed, for example, of polyvinylidene di-fluoride (PVDF). This arrangement is provided with power leads and is subsequently accommodated in a retaining frame.
A pyrodetector having the recited features is disclosed in detail in terms of its structural design and, in particular, in terms of its function, in EP-B No. 1 0 023 354. This European patent corresponds to German Pat. No. 29 30 632 together with German OS No. 30 28 252 and also corresponds to U.S. Pat. No. 4,404,468, incorporated herein by reference. FIGS. 6 and 8 and the corresponding portions of the specification as well as claims in the European patent illustrate and explain embodiments of the sensor element arrangement wherein the permanent orientation polarizations of the plastic film are isodirected for the individual sensor elements. In the embodiment shown in accordance with FIG. 6 therein, however, a common cooperating electrode is shown for two elements, whereas only two sensor elements are shown in the embodiments of FIG. 8 therein.
Although three sensor elements are shown in FIG. 3 of the European patent and are set forth in the specification, these three sensor elements only have respective common electrodes on each side of the plastic film.
The effect of the detector is all the better, the more the heat radiation not to be detected is eliminated from the actual detection event by further sensor elements. This detection event is further intensified when these further sensor elements have a polarization direction opposite that of the sensor element located at the focus. In order to realize this, it can be derived from the European patent to either impress an opposite polarization at the corresponding locations in the plastic film for the sensor elements and to separately connect a respective electrode to the evaluation circuit, or to employ a sensor film which exhibits isodirected orientation polarization over its entire area, and to also individually connect the electrodes to the evaluation circuit in this case as well.
Although these embodiments are fundamentally possible and have likewise been realized in laboratory models, it has nonetheless been found that the fabrication-oriented expense for manufacturing such pyrodetectors is extremely high, so that considerable costs arise which do not yield a rational selling price, particularly because mass production can hardly be realized.